System for and method of modulating magnetrons



Feb. 21, 1939. K. FRITZ SYSTEM FOR AND METHOD OF MODULATING MAGNETRONS Filed Jan. 8, 1936 P- OPT/MUM 1 01 m 65 l4- OPf/Ml/M VON/I65 ATTORNEY.

N ew M 5 g Y B m m Q B a 1 n A 7 1 4MWH1N P K 3% W r J M w .H. A m F N h Patented Feb. 21, 1939 UNITED STATES PATENT many Application January 8, 1936, Serial No. 58368 Germany December 29, 1934 4 Claims. (01. 179-171) The present invention relates to a method of modulating magnetrons.

According to known methods of modulating a magnetron certain difficulties have been encountered chiefly due to the fact that frequency variations in the oscillations produced by the tube itself result from the application of modulation potentials to the field coil by which the electron stream withinthe tube is influenced. If the high frequency oscillations are reduced in frequency beyond certain limits a surge of direct current anode potential is occasioned, and this, in turn,

subjects the tube to an excessive current such, perhaps, as to burn it out.

In order to overcome this disadvantage I have developed-a system which provides for modulating the. magnetic field so as to direct the electrons in spiral paths more or less along planes which are perpendicular to the axis of the oathode-anode system. I then simultaneously modulate the plate circuit in such a manner that a.

tendency of the magnetic field to direct the electrons in helical paths is opposed.

In attempting to carry through a modulation on the ascending left hand branch of the oscillating current-plate potential characteristic serious difficulties are met, since in this region the oscillations break at relatively high amplitudeswhen the plate potential decreases bya small value (Figure 1).

The optimum high-frequency energy not only depends upon a certain plate potential, but also upon the angle between the principal magnetic field and the tube axis. Generally speaking, it depends essentially upon the direction of the magnetic field with respect to the electron paths. Optimum energy for a given tubewill be obtained when the magnetic field is at right angles to the electron paths, or, in other words, if the angle a between the magnetic field and the symmetry aixs of the electrode system equals zero. In maintaining this condition as regards said angle 5 one obtains the strongest oscillating current Is in relation to the plate amperage Ie at otherwise optimum operating conditions (Figure 2). To understand the following descriptions Figuresl and2 may be compared with each other. 50 In the following description reference will be made to the accompanying drawing in which:

Figures 1, 2 and 3 are characteristic curve charts which are referred to in explaining the principles of my invention:

55 Figure 4 illustrates the control of electronic tential and when oz=0 the electrons describe coursesin planes at right angles to the direction of the magnetic field. The maximum radius vector a describing a curve P of approximate heartshape, is slightly smaller than the radius r of the plate having, for instance the shape of: a 15 circular cylinder (Figure 4). Under the abovestated optimum conditionsthe electrons approach the anode, or anode parts AnAz up to a short distance (ra). The periodic distribution of the electrons, first on one of two anode segments 20 and then on the other, results in the generation of high frequency alternating potentials. In the output circuit of the magnetron tube, these alternating potentials are superposed on the direct current anode potential. It is these highfre- 26 quency alternating potentials which control the electrons so that they are carried beyond the radius a.

Nowif, due to the superposed modulation potentials M,-the eifective plate potential decreases 30 substantially,- the maximum diameters of x the electron courses become too small, and the highfrequency control potentials cannot overcomethe pathdifierence (ra) so that at relatively high amplitudes the oscillations suddenly rupture. 35

Attempts have been made to provide modulation on the left hand branch-of the oscillating current-plate potential characteristic by choosing'the angle u different from zero thereby 0b-' taininga course ofthe-oscillatory current" in a0: 40 cordance with curves b and-c of Figure3. It can be readily seen from Figure 2 thatat each angle a different from zero, the plate current in creases rapidly while on the otherhand, according-to Figure 3", the high-frequency energy drops rapidly. Such measures are inadvisable where high oscillation energy and a favorable efficiency is desired.

The present invention aims at obtaining a modulation" at relatively high oscillation, i. e., atfavorable efiiciency'. The idea of the invention is as follows: At high'res'ultant plate potentials, the'electrons describe level courses, while atlow resultantplate potentials, distorted courses are described by the electrons." Hence, if modulation '55 potentials iEM are superposed on the plate potential Ea, the electrons at maximum resultant plate potential Ea+EM move in planes that are at right angles to the symmetry axis of the electrode system. For the resultantminimum plate potential EaEM the conditions for the movements of the electrons should be altered in such a way that rupture of the oscillations is avoided despite a low resultant plate potential.

In the foregoing discussion no mention was made of the fact that at the same time highfrequency alternating potentials are superposed on the potentials of the plate segments, namely always two cooperating plate segments or groups thereof are superposed in opposite phase. At amplitude modulation the amplitude of these alternating oscillations controlling the performance of the high-frequency oscillations, obviously fluctuates, and in order to avoid rupture of the oscillations the influence of these relatively small voltage values is likewise to be considered.

' -A distorted electron course is produced if in the discharge path of the tube an unsymmetrical condition or inhomogeneity with respect to an electrical or magnetic field is produced. The electrons in this case no longer move along a level course at right angles to the cathode but they more or less take a helical course about the magnetic field lines and thus they reach again. the required vicinity of the anode despite a lower anode potential. If the resultant magnetic field has a cross field component, for instance, by the provision of a field inclined towards the principal field at an angle oz, then the electrons describe courses having a cork-screw shape, with the axis of the helix inclined towards the axis of the electrode system. In physics this denotes that the setting up of oscillations has been rendered smooth.

' Such temporarily efiective unsymmetrical conditions or inhomogeneities may be obtained in various ways. Some possibilities will be described with reference to Figures to 7..

Figure 5 shows a modulation circuit in which 5*the modulation potentials of a source M are applied in the known manner to a magnetron having a double split anode whereby the said modulation potentials are superposed on the plate potential Ea, by means of a transformer Ta. In order to afford variation within narrow limits of the angle or between the symmetry axis of the electrode system and the resulting magnetic field, cross field coils Q are provided to which likewise modulation oscillations of the same source M are applied across a transformer T In such symmetrical circuits it is obviously necessary to choose the working point with respect to the angle having the value oc=0 in such difierent manner that the condition a=0 will be attained only at maximum high-frequency amplitudes. The preliminary setting of the rest angle azis obtained in a circuit according to Figure 5, either by means of a continuous direct current with a battery V or in a still simpler manner by a geometrical twisting of the symmetry axis of the tube with respect to the direction of the main magnetic field.

Figure 6 shows a modulation circuit in which, in a manner known as such, the modulation potentials of a source M are superposed on the plate potential Ea, by employing a transformer Ta. For additional modulation, auxiliary magnet poles Hm are provided which may or may not be connected to the principal magnetic field. By means of the circuit herein shown, inhomogeneities of the principal magnetic field are obtained periodically in the rhythm of the modulations. Each auxiliary magnet pole Hm has two windings W1, W2 in the example shown. The one Winding is connected to a direct potential source B1 and assures a preliminary magnetization, the other winding W2 is connected to the modulation source M. The auxiliary direct field intensities and the auxiliary alternating field intensities obviously are to be so chosen that at maximum modulation amplitudes, i. e., at maximum high-frequency energy the additional fields just compensate each other, so that at the optimum anode potential all electron courses are again approximately uniform and level.

The circuit, according to Figure 6, can be considerably simplified by providing the auxiliary pole with but one, but suitably dimensioned winding and placing the same in series to the modulation transformer Ta. This measure utilizes the fact that the emitting current changes with the highfrequency energy, 1. e., in this case also with the modulation amplitude. The direction of the turns of the coil is hereby to be so chosen that in considering the preceding explanations, the desired result will be obtained.

In Figure 7 a modulation circuit is shown in which again, in the manner known as such, the modulation potentials of a source M are superposed on the plate potential Ea. The tube utilized in this circuit is provided with auxiliary electrodes He brought out while insulated against the tube wall. In order to obtain an electrostatic bias, the auxiliary electrodes are connected to the cathode across a suitable direct potential source. The polarity and the size of the source of the biasing potential are to be so chosen in view of the superposed alternating potentials, that at maximum high-frequency amplitudes the bias having modulation frequency and the constant bias just compensate each other.

The lead-ins to the anodes and auxiliary electrodes include the windings of a transformer T serving for the superposition of the modulation potentials. on the electrode potentials. In this circuit, the electro-static bias of the auxiliary electrodes likewise must be so chosen that the actions of the auxiliary direct fields and of the auxiliary alternating fields compensate each other at maximum amplitudes.

The idea of the present invention is not limited to the examples herein shown. Thus it may be found possible to carry out the invention by using only a single auxiliary coil, and a single auxiliary electrode.

I claim:

1. The method of modulating the output energy of an electron discharge tube of the magnetron type wherein the normal flow of electrons is from a linear cathode outwardly in planes perpendicular to the axis of said cathode toward first one and then another of a plurality of cylindrically segmented anodes, which comprises subjecting the electron stream of said tube to the influence of a steady magnetic field having lines of force substantially parallel to the axis of said cathode, subjecting said stream also to the infiuence of a modulated magnetic field having lines of force lying at an angle to the axis of said cathode, and producing said modulated magnetic field by a current which is simultaneously applied between said cathode and said anodes.

2. A modulation system for a magnetron discharge tube having a linear cathode and a plurality of surrounding cylindrically segmented anodes, which comprises means for producing a steady magnetic field substantially parallel to the axis of said cathode and of such strength as to cause electrons to barely graze the inner surfaces of said anodes, means including a modulating source of energy and a magnetic coil energized by said source for deflecting said magnetic field away from parallelism with respect to said cathodeaxis, means for mounting said coil with such orientation that the deflection component of said magnetic field is a function of the energy variations of said modulating source, and means under control of said modulating source for continuously adjusting the voltage applied between the cathode and the anodes to a value such as to maintain constant the oscillation frequency of said tube.

3. A modulation system for a magnetron discharge tube having a linear cathode and a plurality of surrounding cylindrically segmented anodes which comprises means for producing attraction of electrons from the cathode towards the anode segments, constant magnetic field producing means for causing the electrons to describe helical paths in planes substantially perpendicular to the cathode axis, an auxiliary magnetic field producing means under control of a modulating source for controlling the electron means, and means including a cathode-to-anode circuit coupled to said modulating source and having therein a direct current source for causing the amplitude of electronic emission in said tube to be controlled by said modulating source, without influencing the frequency of oscillations in said tube.

4. A magnetron oscillator system in combination with a modulation source, said system comprising an electron discharge tube having a cylindrical arrangement of anode segments and a linear cathode centrally disposed within said arrangement, a primary magnetic field producing means for directing lines of force through said tube in a direction substantially coaxial with respect to said cathode, a secondary field producing means under control of said modulating source for deflecting said lines of force angularly with respect to said cathode axis, a cathode-toanode circuit having a direct current potential source, means including an impedance interconnecting said anode segments for producing oscillations in said tube, said impedance being symmetrically disposed in said cathode-toanode circuit, and means controlled by said modulation source for continually adjusting the cathode-to-anode potential to a value such that the frequency of oscillations is maintained sub stantially constant.

KARL FRITZ. 

